1. Field of the Invention
The invention relates to a power amplifier and more particularly, to compensating distortions appearing in the output of the power amplifier.
2. Background of the Related Art
Typically, a nonlinear distortion appears in the output of a power amplifier. To remove such distortion, an active distortion signal generating circuit is often implemented in a linearizer. FIG. 1 shows a general construction of an active distortion signal generating circuit and FIG. 2 shows a general linearizer connected to a power amplifier.
Referring to FIG. 1, the active distortion signal generating circuit is divided into a linear path and a nonlinear path. The linear path includes a first signal attenuator 2 and a first amplifier 3 for amplifying and outputting a signal having the same phase as the input signal. The nonlinear path amplifies and outputs a signal having a phase difference of 90.degree. from the input signal. The separation of the input signal into a signal of the same phase and a signal having a phase difference of 90.degree. with respect to the input signal is performed by a hybrid coupler 1.
Furthermore, the nonlinear path includes a second amplifier 4 that amplifies one output signal of the hybrid coupler 1. The nonlinear path also includes a second signal attenuator 5 connected to the output terminal of the second amplifier 4. The second signal attenuator 5 generates a distortion signal having a characteristic similar to a distortion signal appearing at the output of the power amplifier (not illustrated). Finally, the output of the second signal attenuator 5 is coupled to the output of the first amplifier 3 by a second hybrid coupler 6, and the coupled signal is input to the power amplifier.
In an ideal active distortion signal generating circuit, components of the input signal should not appear at the output. Only the distortion signal generated through the nonlinear path should be output. Also, the hybrid coupler should separate the input signal into the two signals having an exact phase difference of 90.degree. while the operation of the linear and nonlinear paths should have the same time delay. In actual application, however, the separated signals do not have an exact phase difference of 90.degree. and the time delays in the operational paths do not coincide.
Referring to FIG. 2, an output of a linearizer 21 is input to a power amplifier 22. The linearizer 21 includes a divider 21a that divides the input signal into two paths, a first linear path for maintaining the input signal and a second non-linear path for generating a counterpart distortion signal to remove the distortion component generated by the power amplifier 22.
The second non-linear path includes an active distortion signal generating circuit 21b as described with reference to FIG. 1, and a variable attenuator and a variable phase shifter 21c that varies the amplitude and phase of the output signal from the active distortion signal generating circuit 21b. The first linear path has a delay line 21d that compensates a time error between the input and the output signals of the variable attenuator and variable phase shifter 21c. The linearizer 21 further includes a combiner 21e that adds the input signal from the second path and the distortion signal from the first path, and outputs the sum to the power amplifier 22. The operation of the linearizer as described above will next be explained.
A signal input to the linearizer 21 is processed by the distortion signal generating circuit 21b. Namely, the signal is processed through the linear path wherein the characteristic of the input signal is maintained, and is processed through the nonlinear path by which the distortion component generated by the power amplifier 22 may controlled. Passing through the delay line 21d, the signal is also input to the combiner 21e. The combiner 21e adds the signal from the delay line 21d to the output signal of the active distortion signal generating circuit 21b and outputs the sum to the line-distortion type power amplifier 22.
As discussed above, the active distortion signal generating circuit 21b uses two paths, the linear path and the nonlinear path to generate the signal having a similar distortion characteristic to the output of the power amplifier 22. By adding the input signal to a signal having a phase difference of 180 .degree. with respect to the input signal, the active distortion signal generating circuit 21b removes the original signal from the output. As a result, only the nonlinear signal appears at the output.
Moreover, the output of the active distortion signal generating circuit 21b is adjusted to have a signal characteristic similar to the output of the power amplifier 22 by the variable attenuator and variable phase shifter 21c. The variable attenuator and variable phase shifter 21b may be implemented within the active distortion signal generating circuit 21b rather than as a separate unit shown in FIG. 2. In any case, the variable attenuator and variable phase shifter 21c adjusts the amplitude and phase errors of the signals passing through the linear and nonlinear paths of the distortion signal generating circuit 21b so that the input signal component does not appear at the output of the active distortion signal generating circuit 21b.
FIGS. 3A to 3D are waveform diagrams illustrating signals appearing at various points of the device 21 in FIG. 2. Namely, FIG. 3A shows a signal at the linear path of the active distortion signal generating circuit, and FIG. 3B shows a signal at the nonlinear path of the active distortion signal generating circuit. FIG. 3C shows a signal when an error in the amplitude and phase exist in the linear and nonlinear paths, and FIG. 3D shows a signal when an error in the amplitude or phase do not exist in the linear and nonlinear paths.
An error in the amplitude and phase may be generated in the operating condition of the linear and nonlinear paths of the circuit or circuit elements regardless of the variable attenuator and variable phase shifter. If such errors exist, the input signal component may appear at the output of the active distortion signal generating circuit as shown in FIG. 3C. In such case, the output signal of the active distortion signal generating circuit including an input signal component is input to the power amplifier, causing the performance of the power amplifier to deteriorate.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.